This invention relates to a constant printing pressure mechanism for a label printing machine and particularly a portable label printing and applying machine. One such label printing and applying machine is of the type in which a pressure-sensitive, self-adhesive label strip, having a backing strip, is printed by the squeezing operation of a hand lever, and the printed label is then peeled from the backing strip and applied to the surface of a desired article. But other types of machines and of imprintable labels can be used with the invention.
In some conventional label printing machines of this type, the hand lever and the printing lever are independent from each other. Both of these levers are often rotatably supported by a single shaft. A constant pressure spring is interposed between both levers and a pressure receiving device, i.e. the platen, is attached to the machine so as to receive the printing pressure of a printing head with a certain force. In such a label printing machine, when the hand lever is squeezed, the printing lever is turned down with the actuation of the constant pressure spring so that the printing head attached to the lever strikes the platen with a constant printing pressure.
Even though constant pressure printing is possible by striking the printing head against the platen, irrespective of the strength of squeezing of the hand lever, the operations following the constant printing action are not satisfactory in the above system. In this system, the printing head returns after the printing, together with the releasing of the hand lever. If the hand lever is squeezed for a long time, the type face of the printing head maintains prolonged contact with the printed label surface. Accordingly, excessive ink is absorbed by the label surface which may blot or blur the ink.
Furthermore, the printing head may bounce on the printed label due to the impact of printing action so that double printing may occur. This would adversely affect the accuracy of printing.